greybus: interface: fix hot-unplug control-connection timeouts

Set an interface disconnected flag when the interface has been
hot-unplugged (e.g. forcibly removed or after a reboot), and use it to
disable the control connection early when deregistering the interface
and its bundles.

This avoids a one-second (default) timeout for every enabled connection
(e.g. one per bundle) at hot-unplug, something which for the default
gpbridge manifest currently amounts to five seconds.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>